A conventional cylinder can only be operated by a particular key. Referring to FIGS. 11 and 12, it can be seen that conventional cylinders comprise a plug (100) and a cylinder body (200) which allows the plug (100) to be placed therein. The plug (100) has a keyway (101) and a plurality of bottom pin holes (102) which are arranged above the keyway (101). Each of the bottom pin holes (102) has a bottom pin (103) and the bottom pins are of differing lengths. The cylinder body (200) has a chamber (201) which is drilled with a plurality of top pin holes (202). Each of the top pin holes (202) is aligned with the bottom pin hole (102) and has a top pin (203) and a spring (204). A key (not shown) has a plurality of cut notches. The key is inserted into the keyway (101) and its notches push the bottom pins (103) upward. If the key is a correct one, then the key may push all the bottom pins (103) to the position in which all the top ends of the bottom pins (103) are at a shear line, so that the plug (100) can be rotated relative to the cylinder body (200) by the key to open the lock. As previously mentioned, every lock can be fitted for one key combination number only. If the user of the lock changes, then the lock must be changed for the protection of the latter user. For example, if an apartment is rented first to person A, and some time later to person B, then person B can not be sure that person A did not duplicate the keys of the locks and keep the duplicated keys. Therefore, the latter renter must change all the locks or cylinders to protect his property. But it is uneconomical and inconvenient for the user to change all the locks or cylinders of the building.